In recent years highly sensitive immunoassay techniques have been developed which detect dilute antigen-antibody reactions. These include radio-immunoassay (RIA) in which a reactant is labeled with radioactive iodine, fluorescence immunoassay (FIA) in which the label is a fluorescent dye, and enzyme linked immunosorbant assay (ELISA) in which a reactant is conjugated with an enzyme which catalyzes a color-forming biochemical reaction after the immunochemical reaction has been completed. In each case, the product formed by the antibody-antigen reaction is separated, for example, by binding the reaction product to another antibody which, in turn, is bound to an immobilized membrane. Once separated, the antibody-antigen reaction product can be detected and analyzed.
The first two types of immunoassay procedure described above, i.e., radio-immunoassay and fluorescence immunoassay, require specialized equipment even for qualitative analysis and therefore cannot be used at home, or in doctors' offices or laboratories that do not have that specialized equipment. The ELISA method enables visualization of the reaction product but often requires multiple lengthy incubation periods to develop adequate color intensity for the visualization. Such a multi-step procedure enhances the risk of error, particularly when the procedure is conducted by an unskilled person.
A fourth type of immunoassay procedure has resulted from prelabeling reactants with colloidal gold. The reaction product has a relatively weak optical density in the visible spectrum and requires enhancement, e.g., by a subsequent silver staining procedure for most visual applications. Because colloidal gold is electron opaque, this method is valuable for use in electron microscopy.